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Generator synchronization

Discussion in 'Electrical Engineering' started by **THE-RFI-EMI-GUY**, Nov 4, 2004.

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  1. I am interested in hearing about simple methods of synchronizing two
    consumer grade standby generators under varying loads.


    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
  2. John Gilmer

    John Gilmer Guest

    My first advice would be: FORGET IT.

    You can sync them by just starting them both and placing them in parallel.
    The one will slow and the other will speed. (I suspect that you may have to
    try to do this more than one. If you close when they are far out of sync
    you will pop the breakers on both machines.

    But without an adjustable governor and some way of monitoring how much power
    each machine is delivering, it will be difficult to share the load.

    The "classic" way of syncing two machines was to place a lamp across the
    contacts of the switch that, when closed, will put the machines in parallel.
    You put a 3/4 load on the first running machine and bright the second
    machine up to speed but slight slow. You "inch up the speed" of the new
    machine and when it is SLIGHTLY faster you thown the switch when the bulb is
  3. SQLit

    SQLit Guest

    Small gennys below ~12.5kw are engine speed governed. Above that line there
    are usually controls that are speed independent. So if you below the magic
    number, buy a bigger gen or split your loads.
  4. Assuming both had an electronic speed control, with constant RPM =
    constant frequency, and a zero crossover switch. would both generators
    share the load, all things being equal?

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
  5. No. You have no control of phase. One will feed the other if out of phase,
    and blow it out. Even if the are EXACTLY on frequency, which they will not
    be, they float.

    DC is a little different you still get one will try to pull all the load,
    and you can get surging which will blow one of them out. .
    But You can add series resistance, I used welding rods one time, (as a
    series resistor) and got the mismatch in current to within 20%(14 volts at
    200 amps)
  6. Don Kelly

    Don Kelly Guest

    Ideally-yes- in practice- no- there always will be small but real
    differences in the speed regulation of the two machines- the net result is
    that the "faster" machine will try to grab all the load--trouble. In
    particular, for constant speed settings on both machines, even if the load
    was equally shared, any change in load would result in one trying to run the
    other as a motor as well as carry the useful load. In utilities, the
    governors (and yes, there have been and still are, very good mechanical
    governors) are set to provide a set speed droop as load increases which
    helps control sharing by providing a definite intersection between the two
    speed- torque curves (or frequency-power). Attempts to raise or lower speed
    on a unit will modify this sharing. With no droop, there is no control of
    sharing of load.

    If you were to try to run two small units in parallel, then it would be best
    to set one to constant speed and adjust the no load speed of the other
    (which would have a speed droop) to determine its share of the load. Any
    load changes would be handled by the "fixed" speed unit. Mechanical
    governors would be adequate but capability to adjust the "no-load" speed and
    the droops would be needed. Is it worth the hassle for small machines?- not
  7. Don Kelly

    Don Kelly Guest

    Utilities have two main controls on the machines- speed and voltage.
    They do not have direct control of phase.
    They survive.

    Adjust the speed of the incoming machine to match the on-line machine as try
    to get the differential voltage as low as possible- there will always be
    some small phase and voltage magnitude difference but, done properly there
    will be no more than a slight "bump' Of course if you try to synch when 90
    or 180 degrees out of phase- then there will be major problems. It's not a
    problem to synchronise but it may be a problem with load sharing due to
    different speed-torque characteristics.
  8. Don Kelly

    Don Kelly Guest

    Note what I said- get the differential voltage as low as possible which
    implies matching voltage and phase as well.
    This doesn't have to be exact. In fact there may even be a small speed
    difference between machines and often is when they are connected. Provided
    that the machines are "close enough" the incoming machine will be pulled
    into synchronisation. If a bit slow, it will be pulled ahead, If fast, it
    will be pulled back.
    As I said- the controls are the speed setting of the turbine and the voltage
    setting of the generator.

    Sure it is ideal to match voltage exactly and match speed exactly and match
    phase exactly. If you can get the voltages between machines =0 and
    unchanging you are there. In practice there will be some error as I
    indicated if you had read rather than jump to invective.

    As to two machines - once synched- how do you think that one changes the
    load balance- it is by trying to speed up one with respect to the other-
    that wont happen but there will be a resulting phase difference between
    internal voltages and, depending on the system, between the terminal

    Also note that I did not say "tie two machines together when they are not
    close enough in phase and magnitude " If you could read, rather than simply
    jump off the deep end, it would help. Certainly I should have said something
    like 30 degrees is too far apart but generally a small phase difference -
    while leading to a bump, should not be a problem.

    Do I know what I am talking about- Yes - on the basis of theory and practice
    I have synchronised machines many times. Sometimes using a synchroscope and
    sometimes using a set of lights.
    And, yes, I have made sure that the phase rotation is right. By the way, if
    this is so, it doesn't matter which you call phase A on one machine and B on
    the other -think about it.

    By the way "synch comes from synchronous which implies the same speed- not
    necessarily the same phase.
  9. Tom Grayson

    Tom Grayson Guest

    WE do this all the time with 3 or 4000 Hp Motors. we found the best way to
    do it is
    have the speed of the incoming motor or generator slightly faster then that
    of the existing system. With this setup the synchronous points come past as
    a steady predictible rate enabling the syncrhonising to be easily performed
    when the phasing is right. Naturally the voltages must match in amplitude.

    a few degrees in error usually do not result in any catastrophic bang and in
    fact, the best results are usually when the closing is done a few degrees
    before the synchronous point, allowing the slowdown of the incoming set to
    be done in these few degrees leading to the synchronous point rather then
    after it.

    There is no need to synchronise phase. in fact, synchronizing phase is
    extremely difficult to do in a dynamic situation because that would require
    your incoming generator to follow all of the small shifts in speed and phase
    that are forever present in a power system.

    I have actually tried to do this in the dim dark ages until we got smart and
    let the system work with a small amount of slip and hit the synchronous
    point as it comes past.
    I can imagine the smaller the generators, the more difficult it would be to
    do the synchronising.

  10. daestrom

    daestrom Guest

    Only in a perfect world, and as we all know, our world isn't perfect.

    For two or more AC machines to share the load well, the governors on the
    machines (either electronic or mechanical) *must* have a characteristic
    called 'droop'. This is the phenomenon where the governor output (fuel
    control/throttle) only rises if the speed is below a setpoint. And in order
    to get more fuel, the speed further below setpoint. Speed droop is often
    measured as a percentage. If the unit is run isolated from all other
    generaters and has its load varied from no-load to full-load, the amount of
    speed reduction needed to have the governor go from cut-off to full throttle
    is measured. This speed reduction (in RPM) is divided by the no-load speed
    (also in RPM) and expressed as a percentage. Speed droops of small to
    medium sized machines often runs 3 to 7 % (i.e. @5%, the speed of an 1800
    RPM, unloaded generator will drop to 1710 when fully loaded)

    If you take two AC machines with the same droop characteristic (let us
    assume 5%), they will share the load quite nicely, even if one is much
    larger than the other and running at a different speed (with different
    number of poles in their respective generators). For example, say you have
    a 10kW, 1800 RPM machine carrying 2kW operating in parallel with a 100kW,
    3600 RPM machine carrying 20kW. Now suddenly switch on a 55kW load. Both
    machines will begin to slow down and their governors will increase their
    fuel flow. But look closely at what happens. The small machine slows down
    about 2.5% to about 1755 RPM. The governor of that machine will increase
    fuel flow by 50% of rating and the generator is now carrying 7kW.
    Similarly, the large machine slows down about 2.5% to about 3510 RPM. The
    governor of *that* machine will increase fuel flow by 50% of rating and the
    generator is now carrying 70kW.

    So, with the same droop characteristic, these two machines were originally
    loaded to 20% of their capacity, and after switching on a large load they
    are both loaded to 70% of their capacity. And it all came about because the
    two governors have similar characteristics. These machines do *not* need
    any cross-connections for this to work. Similarly, a sudden reduction in
    load will come off the two machines nicely and neither will reverse power
    the other unit.

    The only problem is that frequency went from 60hz original, to 58.5hz. This
    can be corrected by adjusting *both* governor setpoints upward at the same
    time. If you adjust just one of the governor setpoints upward, it will
    sense that engine speed is further away from the setpoint and increase fuel
    flow to the engine. This will increase the load on the generator. As the
    load is 'picked up' by this machine, the opposite machine will see a
    reduction in load and start to speed up slightly. So the second machine's
    governor will sense speed closer to the setpoint and decrease the fuel to
    its engine and its generator load will decrease.

    I've paralleled hundreds of generators over the years using both
    synchroscope and lights (three phase lights can be fun as they can be wired
    to 'rotate' :). Almost universally, the 'incoming' machine is run slightly
    faster than the system it is being connected to. This helps to ensure that
    the moment it connects, its governor will see a slight drop in speed and
    increase fuel flow. This is important as most generators intended for
    parallel operation have reverse-power protection and in order to avoid false
    tripping of the unit, it is best to pick up some load immediately. Even
    extremely *large* units (1200MVA), when first connecting in are run slightly
    fast (after all, if the speed is exactly matched, the synrchoscope doesn't
    rotate at all).

    Now, that all said, there is yet a different situation with some of the
    small portable units available. Some of the small portable units are
    actually DC generators with electronic inverters. Since the machine is
    actually DC, all this previous talk about governors and droop is not
    applicable. What is important when connecting inverters together is how the
    inverter electronics are designed. Some manufacturers (Honda I believe is
    one) have a special cable to connect between the units. This cable actually
    connects between the *inverters* so the two *inverters* that are inside the
    units can communicate and share load between them.

    P.S. DC generators can be made to share their loads in a similar manner by
    designing the voltage regulators to have a 'droop'. Or just the machine's
    inherent voltage regulation, if properly designed, will do it as well.
  11. Thanks for the explanation. I didn't know about Honda having a cross
    connect. I will look into that feature.

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
  12. torresD

    torresD Guest

    Sorry, you are wrong, Synchronise mean "phase locking".
    If not phase locked, then how much current flows from one machine into the
    I do it all the time with two 500KW generators at work, you get them close,
    then use a phase lock to keep them together.
    (the arcing that occures gives on a healthy respect of electricity)
    He needs to something else with his little generators, run seperate
  13. torresD

    torresD Guest

    The phase locking is automatic, very difficult to do by hand anyway. |
  14. Wicket gates control the water flow to the turbine, driven by a large
    hydraulic cylinder. I don't have exact numbers but at Manitoba Hydro's
    "Limestone" generating station the units are about 130 MW each, and the
    hydraulic cylinder looked to be about a 14 or 15 inch bore - governor
    oil systems run about 1000 to 2000 PSI.

  15. Don Kelly

    Don Kelly Guest

    Don Kelly

    remove the urine to answer
  16. Don Kelly

    Don Kelly Guest

    Synchronism is not "Phase locking" which implies a set phase angle
    difference - preferrably 0 on the initial connection. This is the objective.
    However the synchronising process involves more than phase
    ocking -relative speed and differential voltage. Certainly, if conditions
    are not exact- 0 phase difference, 0 speed difference and 0 voltage
    difference at the time of connection- there will be a bump - The machines
    are more forgiving than you imply.
    A utility would have no load control on its generators in a phase lock
    situation but I assume that you are referring only to the instant of
    As far as synchronising by hand, this is not a difficult process. It is
    easier to do it with a synchroscope than with a set of lights but in either
    case it is not difficult. In fact a good operator can synchronise an
    incoming machine to the system faster than an automatic control because he
    can anticipate and allow for reaction time etc. The automatic synchronisers
    dither around to try to get things "just right"
    This was touched on by others where the incoming machine is a bit "fast".

    As for Phil's comment- yes, a synchroscope does indicate phase (not exactly
    a precision instrument) as well as relative speed. It does not indicate
    voltage differences. Is a precision circuit needed- no. Is common sense
    needed- Yes.
  17. Don Kelly

    Don Kelly Guest

    Having 2 or more generators on line with one incoming machine (and you are
    synchronising one at a time) will make things easier. The incoming machine
    is then relatively smaller and has less impact on the system.
    Hydor machines are not a problem- yes it can take time to adjust speed but
    the process of nearly matching speed and switching as it approaches the best
    point works well. Still I have seen a 60MW machine come up from start, on
    remote, automatic, control and pick up load within 1-2 minutes (most of
    which were due to the synchroniser dithering).
    Don Kelly

    remove the urine to answe |
  18. Don Kelly

    Don Kelly Guest

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